feat: calculate MW-miles for transmission-expanded change tables

[danielolsen]

This branch adds a new script analyze/mwmiles.py to calculate the aggregate MW-miles of upgrades from a given Scenario object, with a corresponding test_mwmiles.py script. This testing functionality uses several testing tools added in the PR for carbon (#39): the tests/ folder in root, the context.py file to ensure that the tests always import the right version of PostREISE, and the mock objects in mocks.py.

[danielolsen]

The unit tests test the individual functions with the mocks. An integration test against the real object can be run as:

from powersimdata.scenario.scenario import Scenario
from postreise.analyze.mwmiles import calculate_mw_miles
scenario = Scenario('130')
mwmiles = calculate_mw_miles(scenario)
print(mwmiles)

[rouille]

I ran the following and got an error:

>>> from powersimdata.scenario.scenario import Scenario
>>> s = Scenario('87')
SCENARIO: base | WesternBase_2016_noHVDC_Final_2019Sep

--> State
analyze
--> Loading ct
--> Loading Western interconnect
Loading zone
Loading sub
Loading bus2sub
Loading bus
Loading plant
Loading plant cost
Loading branch
Loading DC line
>>> from postreise.analyze.mwmiles import calculate_mw_miles
>>> a = calculate_mw_miles(s)
--> Loading Western interconnect
Loading zone
Loading sub
Loading bus2sub
Loading bus
Loading plant
Loading plant cost
Loading branch
Loading DC line
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/brdo/REM/PostREISE/postreise/analyze/mwmiles.py", line 46, in calculate_mw_miles
    upgrades = _calculate_mw_miles(original_grid, ct)
  File "/Users/brdo/REM/PostREISE/postreise/analyze/mwmiles.py", line 68, in _calculate_mw_miles
    upgraded_branches = ct['branch']['branch_id']
KeyError: 'branch'

This is the base case scenario and hence branches are not upgraded, i.e., ct['branch'] throws you an error. I guess we could add a if or try here to handle that as you did in interstate/intrastate:

if 'branch' not in ct or 'branch_id' not in ct['branch']:
        return upgraded_branches

[danielolsen]

Good catch @rouille, I added an if/return to handle this case.

[rouille]

One more